1. Field of the Invention
The present invention relates to metal halide vapor discharge lamps, and, more particularly, to an arc tube construction and discharge operating composition which improve the lamp performance.
2. Description of the Prior Art
Metal halide lamps generally comprise an inner envelope or arc tube which is enclosed within a vitreous outer envelope or jacket. The inner envelope or arc tube contains a quantity of mercury, an inert starting gas and a quantity of one or more metallic halides. Lamp color and efficacy, or lumens per watt, of metal vapor discharge lamps are affected by the vapor pressure of the halides in the arc during operation, which is affected by wall temperature of the arc chamber. Wall temperatures of the arc chamber are determined by the input power to the arc chamber, the geometry and orientation of the arc chamber and the design of the arc electrodes. Wall temperatures are also affected by convection speeds and flow patterns, arc radiation efficiency and the amount of wall coverage by metal halides. Condensation of metal halides and accumulation of the condensate at a relatively cool location within the arc tube reduces lamp efficiency and can result in sudden vaporization of condensate, flaring, which adversely affects color rendering in the operating lamp. Condensation of the metal halides in the end chambers, particularly in the regions where the electrodes emerge from the vitreous material, is undesirable in that the temperatures of these regions is strongly affected by manufacturing variations, causing corresponding variations in the metal halide vapor pressures and thus also in lamp performance. Preventing accumulation of condensate close to the electrodes is therefore an aim of metal halide arc tube design when close control of color is desired. In order to provide a particular lamp color and efficacy in terms of lumens of light output per watt of electricity consumed, various attempts have been made in the prior art to shape the arc chamber and select the discharge medium materials to allow flexibility in orientation of the lamp during normal operation and in selecting the color temperature of the lamp. In standard lamps, large differences in halide vapor pressure exist between horizontal and vertical operation, primarily because of the differences in the condensed halide temperatures in these orientations. In horizontal operation the arc floats to the top of the arc tube, and condensate tends to accumulate on the lower arc tube wall, which is further from the arc and cooler than the walls and end chambers in vertical operation. The result is generally a marked reduction in halide vapor density for horizontal operation and consequently for the radiating metal densities in the arc, which result from thermal dissociation of the metal halides. The reduced metal densities generally result in increased color temperatures and decreased efficacies as compared to vertical operation. Further, in certain applications, for example the illumination of an object, such as the Statue of Liberty, which limits the positioning of lamps for illumination, a lamp operable at orientations such that the arc tube is oriented other than vertically is highly desirable.